by Professor Gene Wengert
Q.
I am getting more and more customers asking about fancy veneers. I recently saw an article about using polyvinyl-acetate (PVA) glues for veneering. As I recall, the article's author suggested coating the back of the veneer, and the substrate surface, with PVA glue. Let it dry thoroughly and then lay it down with a hot iron. The heat would then melt the two surfaces together. Could you please clarify and further explain this procedure? Thank you so much!
A.
"Old fashioned PVA glues" (that is, the white glues) will soften with heat and re-harden when cooled. So, the idea of veneer in the manner described is good. It is especially effective because we do not need high strength from the resulting bond. The resulting bond is low in strength. (The new PVA glues that provide water protection and form a cross-linked bond do not work so well.)
There are some risks--foremost might be the risk that when heat is applied, the adhesive, when it becomes fluid will penetrate into both the veneer and the substrate, creating the risk of a starved joint (one low in glue between the pieces) and also resulting in glue traveling through the veneer into or onto the good face (called bleed through). This is likely especially when too much heat or too long a heat treatment is used. On the other hand, we need adequate heat to assure that the glue files all the nooks and crannies and that the two pieces of wood will become close enough to form a good joint. Plus, you need to keep the full pressure on the joint after the heat source is removed, while the glue re-cures, to avoid a gap or thick joint.
But let me repeat: the process works if we don't need a glue bond with full strength.
I'll bet you could use double faced "Scotch" tape too.?!?? How about a hot melt adhesive instead--it is a good gap filler, compared to PVA, and will set more quickly?
Incidentally, what if you do this with a dining room table top, wouldn't the hot dish put on the table at Thanksgiving dinner melt the glue then, and when the dish is removed, might we not have a slight delamination problem?
As an old-timer who either preaches or reminisces, be careful with PVA laminated tops.
Professor Gene Wengert is Extension Specialist in Wood Processing at the Department of Forestry, University of Wisconsin-Madison.
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Comment from contributor N:
My experience with using an iron and dried PVA glues to adhere veneer to a substrate have come up with a couple of big plusses that Professor Wengert forgot to mention. The first and biggest is that you don't have any oozing glue coming out the sides to worry about. This is reason enough to use this method whenever I can and the project's end use warrants it.
The second plus comes in when you are working with a burl or figured wood veneer where the sheet is so wild and far from flat that it looks like the Rocky Mountains. The heat from the iron helps to tame the sheet by softening and flattening it (probably activating the bending properties of the wood, but I am not sure and Professor Wengert can probably elaborate on this), usually without any problems (like cracking).
One big downside that was not mentioned that I found out the hard way is you have to take into account that many veneers, mainly burls, figured wood veneers, and open grained or non-tight grained wood veneers will have a tendency to shrink a little from the heat of the iron. In many cases this is not a significant factor, because the veneer is usually overlapping the edges of the substrate. On the other hand, if you are trying to piece together book-matched pieces of veneer or are butting the veneer up to something that it needs to be flush with, this is a very important consideration.
As far as temperature is concerned, I have found that the cotton setting (about 400 F), which is around #4 on the dials of my irons, is the best temperature to set the iron to for this method of adhering veneer. This is usually the highest heat setting below the steam range of settings. If you choose to use a higher temperature, I do not recommend using steam, with the exception of a situation where you are working with a wild piece of veneer that is far from flat and is not being tamed by the heat of the iron alone (this should be very rare). Be careful, because the steam can do all sorts of bad things, like cause the glue to bleed through, shrink the veneer and/or disfigure it, cause colors to bleed or cause the veneer to fade or discolor.
